The invention relates to a brushless d.c. motor having a commutation circuit for the commutation of phase currents in order to generate a given commutation angle in a given mode of operation of the d.c. motor.
Motors of this type are used, for example, in domestic appliances such as food processors.
From EP 0 762 625 A1 a brushless d.c. motor is known, wherein a smoothed d.c. supply voltage is applied to a commutation circuit comprising three half bridges with two switching elements each. The commutation circuit supplies phase voltages for the three phase windings of the motor stator, which are connected in star arrangement. A module comprising control and regulation elements supplies commutation signals to the commutation circuit, which signals define the switching states of the switching elements of the commutation circuit. For this purpose, the module processes sensor signals which correspond to the rotational position of the d.c. motor and which are supplied by position detectors. The position detectors are arranged in the proximity of the rotor and, for example, take the form of Hall sensors. The waveforms and the magnitudes of the phase voltages are adjusted by means of pulse width modulation (PWM). The pulse width modulation is realized by rapid switching-over of the switching elements of the commutation circuit, the frequency of the pulse width modulation being substantially higher than the commutation frequency.
Brushless d.c. motors in which, for example for reasons of cost, the commutation angle is not freely adjustable but in which only a limited number of commutation angles are available, have the problem that a satisfactory operation of the motor is not guaranteed for all operating conditions.